New process for casting rocket propellant grains



United States Patent 3 Claims. (Cl. 264-3) (Granted under Title 35, US. Code (1952), sec. 266) The invention herein described may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

The present invention relates to a new process for casting rocket propellant grains and particularly, a process for casting rocket propellants of gelled hydrazine fuel formulations.

In the field of rocket propellants it has been the general practice to pour the fuel gel into a motor with a mandrel in place, then force a screen into the propellant. This method can only be used on very small motors, 2 /2" or so. Very often the material gelled too fast and prevented the insertion of screen or other reinforcement material. Other times constituents in the gel, such as powdered metal, would settle out due to too low initial viscosity. The present invention overcomes the difficulties encountered in the old method described above by a process whereby csasting at relatively low temperatures improves the propellant characteristics.

An object of the present invention is the provision of a method for manufacturing a propellant grain which is simple and will increase the quality of the grain.

Another object is to provide a method for loading rocket motors at a temperature just above the freezing point which provides for longer pot life.

A further object is to provide a propellant with uniform density and increased energy output.

Other objects and advantages of the invention will be readily appreciated as the same become better understood by reference to the following detailed description:

The general operating procedure for mixing and casting hydrazine gels will be used to illustrate the present invention, but should not be considered as limiting the invention.

The mixing technique for the hydrazine gel systems containing metal powder additives is fairly simple. A typical gel consists of the following:

Ingredients: Weight percent Hydrazine (liquid) 40.0 Tetramethylguanidine (liquid) 0.1 Aluminum (powder) 45.0 Sodium carboxymethylcellulose (solid) 12.0 Antimonyl potassium tartrate (solid) 2.9

The mixing technique for the hydrazine gel system containing the metal powder additives as above set out was fairly simple. The solid materials were blended thoroughly, and then the liquids, which had also been mixed, were added. The mixture was stirred until homogeneity was obtained. Viscosity increases in the system as the sodium carboxymethylcellulose (CMC) gradually swells. After about 30 minutes the viscosity is sufficiently high to prevent settling of the aluminum particles. It was found that hydrazine prethickened with a small amount of sodium carboxymethylcellulose before being mixed with the other materials, causes a more rapid suspension of the metal additives before much swelling of the additional sodium carboxymethylcellulose (CMC) takes place. This rapid suspension method aids in producing a mix with a longer pot life. About 50 minutes is available for casting and other processing.

ICC

A rocket motor was prepared beforehand with aluminum screen reinforcement. About 25 layers per inch of 30 x 30 mesh, S-mil aluminum screen were stacked in alternating layers of flat wire and crimped wire. The stack was rolled to the proper diameter to form a grain skeleton. This was fitted in place in a rocket motor and the gelled hydrazine formulation was cast into the motor, using a vibrator to speed the flow of gel into the motor. This step took place at around freezing temperature (32 to 40 F.). freezing temperature provides a propellant having increased pot life and uniform density. The rocket motor was cured at ambient temperature for a couple of Weeks. Maximum tensile strength of the material produced by this method was about p.s.i.; and the modulus around 20,000 p.s.i./in./in.

Many hydrazine gel propellant samples containing the 30 x 30 mesh screen which had the crimped screen running axially were tested and they sheared at a stress of about 100 psi. The same gel formulation having the ribs of the crimped screen running at right angles to the length of the rocket motor did not shear but were deformed enough under the stresses applied to allow the sample to pull through the test holder. Therefore, it is concluded that the alignment of the crimped ribs affects the directional strength of the gels. The number of screen layers in the grain appears to also affect the directional rigidity.

Other hydrazine gel formulations may be used. Work has progressed on finding a crosslinking agent which makes the gels stronger and more suitable for propellant application. It has also been found that to increase the basicity of the system by adding amines such as unsymmetrical dimethylhydrazine, tetramethylguanidine and triaminoguanidine effectively increase the pot life.

This is the first time to applicants knowledge that temperatures lower than room temperature and near the freezing point have been used to load rocket motors. A temperature just at or slightly above the freezing point is most desirable. The advantages of the present process are longer pot life, high initial viscosity, no screen deformation, and the elimination of bubbles in the propellant gel. The longer pot life due to the low temperature operation increases the allowable time for loading. This results in better quality motors by reducing the possibility for human error, increasing reproducibility and allowing better quality control. The higher initial viscosity due to the lower temperature prevents settling of solids from the gel mix and gives uniform density throughout the motors. Furthermore, there is no screen deformation since the reinforcing material is not forced into the gel; also, there are no bubbles in the cured propellant because of the longer pot life and vibration.

The process described herein could be easily made continuous by feeding the ingredients slowly into a container while withdrawing an equal amount of mixed gel off. The mixed gel could then be fed continuously into motors in an open flow system or under pressure.

A few tests were made using aluminum wool as the reinforcing agent. Obviously, metal screen of the various metallic elements can be used as a reinforcing material and also for the purpose of increasing energy output of the propellant formulation.

It should be noted that work with hydrazine is dangerous. Hydrazine is a toxic liquid and precautions must be taken against breathing and skin absorption.

Obviously many modifications and variations of the present invention are possible in the light of the teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

The critical step of casting at or near I What is claimed is:

1. The process for casting a propellant grain consisting essentially of hydrazine comprising:

(a) forming a screen wire skeleton within a container;

(b) casting gelled hydrazine at about 32 F. around said skeleton; and

(c) curing at room temperature for about two weeks.

2. The process for making a reinforced propellant grain consisting essentially of gelled hydrazine comprising:

(a) positioning aluminum screen wire skeleton within a rocket motor casing;

(b) pouring, while using a vibrator, gelled hydrazine around said skeleton at a temperature ranging from 3240 F.; and

(c) curing at ambient temperature until a firm propellant grain forms.

3. The process for making a reinforced rocket motor propellant grain consisting essentially of hydrazine comprising:

(a) positioning layers of crimped screen wire in a rocket motor casing so that the ribs of said wire are forms.

References Cited by the Examiner UNITED STATES PATENTS 8/ 1 961 5/ 1962 2/ 1963 l/ 1965 4/1965 5/ 1965 Webb FOREIGN PATENTS 8/1959 Australia.

Rice

20 LEON D. ROSDOL, Primary Examiner.

L. DEWAYNE RUTLEDGE, Examiner.

Haymes et a1 Long Hsieh et al. Forsberg et al. 

1. THE PROCESS FOR CASTING A PROPELLANT GRAIN CONSISTING ESSENTIALLY OF HYDRAZINE COMPRISING: (A) FORMING A SCREEN WIRE SKELETON WITHIN A CONTAINER; (B) CASTING GELLED HYDRAZINE AT ABOUT 32*F. AROUND SAID SKELETON; AND (C) CURING AT ROOM TEMPERATURE FOR ABOUT TWO WEEKS. 